Right from the moment of birth, women face a ticking clock, counting down to the end of their life’s fertile phase. In their fourth month in the womb, their immature ovaries begin to develop primordial follicles, the structures that will eventually give rise to egg cells. At birth, each ovary has around 400,000 follicles and won’t make any more. During each menstrual cycle, around a thousand of these cells become activated per ovary. By the time a woman goes through menopause, she has less than a thousand left and her chances of being a biological mother are slim to none.

Follicles stay in a dormant phase that can last for months or even years, until they are gradually activated. Now, a team of Chinese, Japanese and American scientists, led by Jing Li from Stanford University, have found a way to activate these dormant cells at will. It’s a step that could help infertile women, or those who freeze their ovaries before cancer treatments, to eventually have their own children.

Li’s work shows that despite their ability to slumber for decades, follicles only need a gentle nudge to awaken. She managed to activate dormant follicles in the ovaries of newborn mice using chemicals that shut off a single gene called PTEN. When she transplanted these clusters into mice whose ovaries had been removed, they developed into mature follicles. From these came eggs that could ultimately be fertilised and develop into healthy pups.

As with most such discoveries, Li’s work hinges on a lot of previous research. In particular, two years ago, Pradeep Reddy from Umea Universit showed that PTEN controls the steady activation of follicles. If mice lack the gene entirely, all of their dormant follicles become activated at once and their entire supply is exhausted in early adulthood. This dramatic switch means that their ovaries fail prematurely. Li wanted to see if she could achieve the same ends in a more controlled way.

Rather than knocking out the PTEN gene altogether, she temporarily blocked it by soaking ovaries from newborn mice in a chemical called bpV(pic). PTEN works by holding back another gene called PI3K, so Li also tried a chemical called 740Y-P, which activates PI3K. In normal ovaries, the unleashed PI3K targets a protein called Foxo3, which is then removed from the nucleus of follicle cells. This is the trigger that activates them, and that’s exactly what Li saw in her chemically treated ovaries. Foxo3 left the building and the follicles matured, particularly if they were transplanted into a living host.

None of the cells ever developed into a tumour, which is a real concern since one of PTEN’s role is to keep cancer at bay. Instead, the activated follicles eventually produced oocytes, the precursors of egg cells, which seemed normal in every important respect. They showed the standard patterns of methylation – chemical ‘Post-it’ notes that add onto genes and affect how, when and where they are activated. When fertilised, the oocytes grew into healthy embryos and eventually into 20 healthy pups. And best of all, these pups were themselves able to bear live young of their own.

Li showed that the same trick might work in humans too, but with more technical challenges. During operations on women with ovarian cancer, she managed to get pieces of ovary containing primodial follicles. She treated the tissues with the same chemicals as before and transplanted them into mice. The result: mature follicles and oocytes. These weren’t fertilised for obvious ethical concerns, but they seemed to show some problems with their nuclei – that will need to be checked in studies using other primates before this technique could ever be used safely in people.